At the beginning the smoke has laminar flow. After a few inches though, it becomes turbulent because the smoke cools down and the air around it starts to affect it more. When hot smoke is released into air colder than it is, it quickly accelerates upward. The hot smoke is lighter than the cold, dense air around it. As the smoke rises, it quickly cools down, folds onto itself, and reaches equilibrium with the room. It then dissipates.